It is hypothesized that the degree to which the G protein, Gs?, is associated with lipid rafts is an indicator of both depression and therapeutic response. The original observations suggesting this biosignature were made in postmortem tissue, and we have extended this work with in vivo and in vitro preliminary data suggesting that raft localization of Gs?, determined by simple detergent extraction, is a biomarker of both depression and antidepressant response. There are suggestions that n-3 PUFA either have antidepressant activity or can synergize the action of some antidepressant drugs. To test this, we will use a glial cell line as well as lymphoblasts from depressed subjects who were either antidepressant responsive or unresponsive. This should provide a predictive model for the response to an antidepressant agent, whether that agent is fish oil, citalopram, or some combination. We will test the applicability of the biomarker by measuring the lipid raft distributin of Gs? in membranes from blood cells collected during an NIMH-funded clinical trial showing n-3 PUFA augmentation of antidepressant response. The proposed experiments also attempt to mesh mechanistic preclinical studies with a clinical study to propose and test a biomarker. The preclinical work seeks to determine whether in-vitro treatment with antidepressants n-3 PUFA shifts Gs? into non-raft fractions of the plasma membrane, where it more effectively activates adenylyl cyclase. We suggest that a compound with antidepressant efficacy concentrates in cholesterol-rich membrane domains (lipid rafts) and disrupts the anchoring of Gs? within those domains. One possible mechanism for this is direct modification of Gs?. This will be studied by biochemical fractionation of membrane components, fluorescence recovery after photobleaching (FRAP) and by cellular imaging. Since most antidepressant therapy (save ketamine) requires a time lag prior to therapeutic efficacy and this can be replicated in cells, translocation of Gs? in response to antidepressant agents will be monitored in real time using a fluorescent Gs? fusion protein. Should n-3 PUFA show these biological hallmarks of antidepressant activity, alone or in combination with SSRIs , this will be translated by examining blood taken in a clinical study of the antidepressant efficacy of n-3 PUFA ( SSRIs). Initial data from this study show a greater antidepressant response to n3 PUFA + SSRI than to placebo or SSRI alone. Finally, we will test, with the biochemical and imaging studies, lymphoblasts, derived from depressed patients with known response (or lack thereof) to citalopram. One goal is to develop a high content screen that might suggest, prospectively, the effectiveness of a given potential therapy. Successful completion of the proposed studies will also indicate the usefulness of n-3 PUFA supplementation to antidepressant therapy for decreasing time of therapeutic onset and/or lowering overall antidepressant dose. They will also pave the way toward establishing a low-cost blood biomarker for both depression and antidepressant efficacy over a broad range of agents.